GB2476228A

GB2476228A - Improvements in security devices

Info

Publication number: GB2476228A
Application number: GB0920308A
Authority: GB
Inventors: Adam Lister
Original assignee: De la Rue International Ltd
Current assignee: De la Rue International Ltd
Priority date: 2009-11-19
Filing date: 2009-11-19
Publication date: 2011-06-22
Anticipated expiration: 2029-11-19
Also published as: GB2476228B; GB0920308D0; WO2011061495A1

Abstract

An improved security device that can be used in for various authenticating or security applications, and in particular to an optically variable security device utilising multilayer polymer films comprises a first multilayer film 12 which comprises alternating layers of at least two different polymers and a second multilayer 13 film which comprises alternating layers of at least two different polymers and which exhibits different reflective characteristics to the first film. At least one of the layers comprises a light absorbing dye or pigment which lies between the first and second films.

Description

IMPROVEMENTS IN SECURITY DEVICES

The present invention relates to improvements in security devices that can be used in for various authenticating or security applications, and in particular to an optically variable security device utilising multilayer polymer films.

The increasing popularity of colour photocopiers and other imaging systems and the improving technical quality of colour photocopies has led to an increase in the counterfeiting of banknotes, passports and identification cards and the like. There is, therefore, a need to add additional authenticating or security features to existing security features. Steps have already been taken to introduce optically variable features into substrates used in such documentation that cannot be reproduced by a photocopier. There is also a demand to introduce features which are discernible by the naked eye but which are "invisible" to, or viewed differently, by a photocopier.

Since a photocopying process typically involves scattering high-energy light off an original document containing the image to be copied, one solution would be to incorporate one or more features into the document which have a different perception in reflected and transmitted light, an example being watermarks and enhancements thereof.

It is known that certain liquid crystal materials exhibit a difference in colour when viewed in transmission and reflection, as well as an angularly dependent coloured reflection. Liquid crystal materials have been incorporated into security documents, identification cards and security elements with a view to creating distinctive optical characteristics. EP-A-0435029 is concerned with a data carrier, such as an identification card, which comprises a liquid crystal polymer layer or film in the data carrier.

The liquid crystal polymer is solid at room temperature and is typically held within a laminate structure. The intention is that the liquid crystal layer, which is applied to a black background, will demonstrate a high degree of colour purity in the reflected spectrum for all viewing angles.

Automatic testing for verification of authenticity is described using the wavelength and polarization properties of the reflected light in a single combined measurement.

This has the disadvantage of being optically complex using a single absolute reflective measurement requiring a uniform

liquid crystal area on a black background.

AU-A-488,652 is also concerned with preventing counterfeit copies by introducing a distinctive optically-variable feature into a transparent window security element.

This document discloses the use of a liquid crystal "ink" laminated between two layers of plastic sheet. The liquid crystal is coated on a black background so that only the reflected wavelengths of light are seen as a colour. The security feature is primarily provided by therrnochrornic liquid crystal materials, which have the characteristic of changing colour with variation in temperature.

Liquid crystal materials can be incorporated into security devices either as a film, as for example in WO-A--3-.

03061980, or in the form of an ink as a liquid crystal pigment in an organic binder, as for example in EP-A- 1156934. The advantage of a liquid crystal ink is that it can be applied using conventional printing processes and therefore it is relatively straightforward to apply the liquid crystal material in the form of a design. However the colour purity, brightness and sharpness of the observed colour and colour-shift are significantly degraded for a pigmented liquid crystal ink compared to a liquid crystal film. This degradation is due to the variability in alignment of the cholesteric helical axis between the individual liquid crystal pigments compared to the uniform alignment of the liquid crystal film.

A method of increasing the range of available colours in liquid crystal films is described in US 4,893,906, Tim which two or more liquid crystal coatings are overlaid to obtain new colours as a result of the colour additive properties of the liquid crystal coatings which do not absorb light. WO-A-2005105474 describes a security device comprising two superimposed cholesteric liquid crystal layers in which the additive mixing of the colours permits a wider range of colourshift effects. In some of the embodiments in O-A-2005l0546 regions exhibiting different colourshifting effects are created by a partial application of one of the liquid crystal layers in localised areas. A partial application of a liquid crystal film is not straightforward and increases significantly the complexity of the production process compared to simply applying one uniform film over a second uniform film.

WO-A-2008/043981 describes an improved security device which incorporates two layers of optically variable liquid crystal materials which have different reflective properties and a partial layer of a light absorbing material between the liquid crystal layers. This creates two optically variable regions having a different appearance.

A disadvantage with the use of liquid crystal films in the security devices described in the prior art is that the production route requires several steps, such as preparing the liquid crystal polymer film on a carrier substrate, and then transferring the liquid crystal polymer film from the carrier substrate to the substrate of the security device.

Furthermore, when used as a security feature of a security thread, the liquid crystal layers are not sufficiently strong to support themselves and need to be appl±ed to a support layer. Liquid crystal materials are also expensive and some forms are not particularly robust and therefore generally need further cover layers to protect them.

Additional layers lead to extra bulk, which is not necessarily desirable in a security feature.

It is an object of the present invention to provide a optically variable security device which overcomes these disadvantages.

The present invention provides a security device comprising a first multilayer film which comprises alternating layers of at least two different polymers, a second multilayer film which comprises alternating layers of at least two different polymers and which exhibits different reflective characteristics to the first film, and a partial first layer of a light absorbing material between the first and second films, wherein at least one of the layers of the second multilayer film incorporates an absorbing dye or pigment.

A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:-Figure 1 is a plan view of a security document incorporating a partially embedded security device of the present invention; Figure 2 is a cross-sectional side elevation of a security device of the present invention; and Figures 3 and 4 are plan views of an alternative embodiment of the present invention viewed in reflected light at normal incidence and tilted away from normal incidence respectively.

It has been found that multilayer polymer films provide a colourshift effect which is comparable to that of liquid crystals, yet are sufficiently strong to be self supporting.

Such films are described in EP-A-1047549 and are formed of multiple layers (hundreds or thousands) of at least two different materials. The various layers may have different actual and/or optical thickness and different indices of refraction. These films contain no dye or pigment or other element which may fade in time, as they just use selective reflection to provide the colourshift effect. As incident white lights strikes the film, light of a specific wavelength is reflected whilst other wavelengths are transmitted through the layers to be reflected at different angles to the normal. Thus when viewed at different angles of light different colours can be seen.

As described in EP-A-1047549 the layers may be selected so that light in the non-visible (infra-red) part of the electromagnetic spectrum is reflected at normal incidence, and the film thus appears substantially transparent. As the film is tilted away from normal it reflects a visible colour (e.g. red).

Referring to Figure 1, the present invention provides a security device 10 for protecting a document of value 11. As shown in Figure 2, the security device 10 comprises a first multilayer film 12 and a second multilayer film 13 which exhibits different reflective characteristics to the first film 12. Each of the multilayer films 12,13 has alternating layers of at least two different polymers. At least one of the layers of the second multilayer film 13, at or adjacent to one surface 15a of the second film 13, incorporates a light-absorbing dye or pigment which substantially prevents the transmission or reflection of at least one wavelength of light. whilst the use of black, or very dark, absorbing dyes or pigments (i.e. which absorbs substantially all of the light) is preferable because it gives rise to the most strong colourshift effects, other effects may be generated by the use of absorbing dyes or pigments of other colours or a combination of colours which only partially absorb light, giving rise to differing apparent colcurshift colours. The first film 12 is laminated to the opposing surface 15b of the second film 13. Thus the dark coloured light-absorbing layer lies at an outer surface of the security device 10. A separate partial layer 14 of a dark light-absorbing material is applied between the first and second multilayer films 12, 13.

The security device 10 can be incorporated into secure documents 11 in any of the conventional formats known in the prior art, for example as patches, foils, stripes, strips or threads. The security device 10 can be arranged either wholly on the surface of the document 11, as in the case of a stripe or patch, or can be visible only partly on the surface of the document 10 in the form of a windowed security thread. Security threads are now present in many of the world's currencies as well as vouchers, passports, travellers' cheques and other documents. In many cases the thread is provided in a partially embedded or windowed fashion where the thread appears to weave in and out of the paper and is visible in windows 16 in one or both surfaces of the document 11. One method for producing paper with so- called windowed threads can be found in EP-A-0059056. EP-A- 0860298 and WO-A-03095188 describe different approaches for the embedding of wider partially exposed threads into a paper or other substrate. Wide threads, typically having a width of 2-6mm, are particularly useful as the additional exposed thread surface area allows for better use of optically variable devices, such as that used in the present invention. Figure 1 shows the security device 10 of the present invention incorporated into a security document 11 as a windowed thread with windows 16 in which the security document 10 is exposed and areas 18 in which the security device is embedded within the substrate of the document 11.

In a further embodiment of the invention (not shown) the device 10 is incorporated into the document 11 such that regions of the device 10 are viewable from both sides of the document 11. Methods of incorporating a security device such that it is viewable from both sides of the document 11 are described in EP-A-1141480 and WO-A--3054297. In the method described in EP-A-1141480 one side of the device 10 is wholly exposed at one surface of the substrate in which it is partially embedded, and partially exposed in windows 11 at the other surface of the substrate.

In the case ofr a stripe or-patch, the security device may be prefabricated on a carrier strip 17 and transferred to the substrate in a subsequent working step.

The security device 10 can be applied to the document using an adhesive layer, which is applied either to the security device 10 or the surface of the security document 11 to which the device 10 is to be applied. After transfer, the carrier strip 17 is removed leaving the security device 10 exposed. Alternatively the carrier strip 17 can be left in place to provide an outer protective layer.

Following the application/incorporation of the security device 10 the security document 11 generally undergoes further standard security printing processes including one or more of the following; wet or dry lithographic printing, intaglio printing, letterpress printing, flexographic printing, screen-printing, and/or gravure printing. In a preferred embodiment, and to increase the effectiveness of the security device 10 against counterfeiting, the design of the security device 10 can be linked to the document 11 it is protecting by content and registration to the designs and identifying information provided on the document 11.

In one embodiment of the invention the first multilayer film 12 is selected to have approximately 140 to 150 layers, each layer having a thickness of approximately 0.1 microns, resulting in a total film thickness of approximately 15 microns. The film 12 preferably has a green to blue colourshift. The partial absorbing layer 14 is applied to one surface of the film l2, preferably by printing a black ink using a suitable printing method such as gravure, and preferably in the form of a design.

A laminating adhesive 19 is applied over the partial absorbing layer 14 and the exposed regions of the first multilayer film 12 to enable the second rnultilayer film 13 to be laminated thereto. The second film 13 has a different structure to the first film 12, preferably having 140 to 150 layers, each layer having a thickness of approximately 0.17 microns, resulting in a total film thickness of approximately 25 microns. The second film 13 preferably has an infrared to red colourshift.

The material used to form the absorbing layer 14 and the dye incorporated into the second film 13 are preferably a pigmented ink or coating or alternatively a non-pigmented -10 -absorbing dye can be used. They may also comprise magnetic ink. Whilst the use of black, or very dark, absorbing materials (i.e. which absorbs substantially all of the light) may give rise to the most strong colourshift effects, other effects may be generated by the use of absorbing materials of other colours or a combination of colours which only partially absorb light, giving rise to differing apparent colourshift colours. The use of a different coloured partially absorbing layer 14 to the colour of the dye in the second film 13 enables the number of optically variable regions to be increased further.

In order to improve the appearance of the reverse side of the security device 10 a full metal layer 20 may be applied thereto by printing with a metal or metal effect ink, for -example an aluminium loaded ink. Alternatively a metal layer may be deposited using an evaporating technique.

Adhesive layers 19 may be applied to the outer surfaces of the device 10 to improve adherence to the secure document 11.

The application of a partial absorbing layer 14 between the two mnultilayer film 12,13 creates two optically variable regions, Regions A and B. In Region A there is no absorbing layer between the two multilayer films 12,13 such that the wavelength of reflected light, at any given angle of incidence, is a result of the additive mixing of the individual wavelengths of light reflected from the two multilayer films 12,13. In Region B there is an absorbing -1]. -layer 14 between the two multilayer films 12,13 and the wavelength of reflected light, at any given angle of incidence, is solely the reflected light from the second multilayer film 13. If the second film 13 is selected to provide an infra red to red colourshift, the film 13 appears substantially transparent at normal incidence.

The designs generated by the partial application of the absorbing layer 14 is preferably in the form of images such as patterns, symbols and alphanumeric characters and combinations thereof. The designs can be defined by patterns comprising solid or discontinuous regions which may include for example line patterns, fine filigree line patterns, dot structures and geometric patterns. Possible characters include those from non-Roman scripts of which examples inc±ude but are not limited to, Chinese, Japanese, Sanskr±t and Arabic.

In a further embodiment one or both of the films 12,13 is a partial layer. where the second film 13, for example, is a partial layer, such that in certain regions the first film 12 is exposed, then a further optically variable region can be created in which the wavelength of reflected light, at any given angle of incidence, is solely the reflected light from the first film 12.

The use of a rnultilayer polymer film where only one component of the colourshift is in the visible region of the electromagnetic spectrum enables an image to be incorporated into the device 10 that only becomes apparent at certain -12 -angles of view. In one example, illustrated in Figures 3 and 4, and referring to the cross-section in Figure 2, the second film 13 reflects light in the infrared region of the electromagnetic spectrum when at normal incidence (Figure 3), appearing colourless and transparent, and reflects red light when tilted away from normal incidence (Figure 4) . The first film 12 exhibits a green-blue colourshift. Regions A and B are defined by the partial dark absorbing layer 14 between the two films 12, 13 which, in this example, is applied in the form of alphanumeric characters such that Region B is a repeating pattern of the words DE LA RUE and Region A is the background. When viewed in reflection and at normal incidence both Regions A and B will appear green due to the transparent colourless appearance of the second film 13 having no visible effect on the appearance of the device 10. On tilting the device 10 such that it is viewed away from normal incidence Region A appears magenta, due to the additive colour mixing from the blue reflected light from the film 12 and the red reflected light from the second film 13, and Region B appears blue due to the reflected light coming solely from the first film 12. To the authenticator the device 10 appears uniformly green at normal incidence but on tilting away from normal incidence the repeating legend DE LA RUE appears in a magneta colour against a blue

background.

The security device 10 can be used in combination with existing approaches for the manufacture of threads. Examples of suitable methods and constructions that can be used include, but are not limited to, those cited within Wa-A- 03061980, EP-A--5l6790, WO-A-9825236, and WO-A-9928852.

-13 -Security devices comprising multilayer polymer films are machine-readable using a spechrometer. The machine readable-aspect of the security device 10 of the present invention can be extended further by the introduction of detectable materials in the absorbing layers 14,15 or by the introduction of separate machine-readable layers. Detectable materials that react to an external stimulus include but are not limited to fluorescent, phosphorescent, infrared absorbing, thermochromic, photochromic, magnetic, electrochromic, conductive and piezochromic materials.

In one preferred embodiment, the light absorbing pigment used in the absorbing layer 14 and second film 13 is machine readable, for example carbon black, to produce a machine-readable or conducting layer. Alternatively it may be a magnetic material, such as magnetite, to produce a machine-readable magnetic layer.

In an alternative machine-readable construction one or more of the light absorbing pigment used in the absorbing layer 14 and second film 13 can be formed using a magnetic pigment, for example magnetite. For example the partial absorbing layer 14 in Figure 2 can be formed from such a magnetic pigment to provide a machine-readable code. In a further embodiment, only part of the partial absorbing layer 14 in Figure 2 is provided with a magnetic pigment and the remainder is provided with a non-magnetic pigment. If both the magnetic and non-magnetic regions are substantially -14 -totally absorbing there will be no visual difference in the liquid crystal layer over the two regions and therefore the format of the code will not be readily apparent.

In an alternative machine-readable embodiment a transparent magnetic layer can be incorporated at any position within the structure of the device 10. Suitable transparent magnetic layers containing a distribution of particles of a magnetic material of a size and distributed in a concentration at which the magnetic layer remains transparent are described in WO-A-03091953 and WO-A-03091952.

In all of the embodiments described, where the finished security document 11 has undergone further standard security printing processes, e.g. litho and intaglio, then the colour and/or design of the images/information on the security device 10 can be correlated to the design of the final printed document 11. The patterns and designs on the device 10 and document 11 may be registered with each other, which makes it very difficult to counterfeit.

Claims

-15 -CLAIMS: 1. A security device comprising a first multilayer film which comprises alternating layers of at least two different polymers, a second multilayer film which comprises alternating layers of at least two different polymers and which exhibits different reflective characteristics to the first film, and a partial layer of a light absorbing material between the first and second films, wherein at least one of the layers of the second multilayer film incorporates an absorbing dye or pigment.
2. A security device as claimed in claim 1 in which one or both of the multilayer films are partial layers.
3. A security device as claimed in claim 1 or claim 2 in which the partial light absorbing layer forms indicia.
4. A security device as claimed in claim 3 in which the indicia comprise one or more design, pattern, symbols or alphanumeric characters or a combination thereof.
5. A security device as claimed in any one of the preceding claims in which the light absorbing material of the partial layer is dark.
6. A security device as claimed in claim 5 in which the light absorbing material used to form the partial layer and in the multilayer film is coloured.
7. A security device as claimed in claim 6 in which the light absorbing material comprises a plurality of colours.

-16 -
8. A security device as claimed in any one of the preceding claims in which the light absorbing material used to form the partial layer and in the multilayer film is a pigmented ink or coating.
9. A security device as claimed in any one of claims 1 to 7 in which the light absorbing material used to form the partial layer and in the multilayer film is a non-pigmented dye.
10. A security device as claimed in any one of the preceding claims in which the light reflected by one or both of the multilayer films at certain angles of view is in the non-visible wavelength of the electromagnetic spectrum.
11. A security device as claimed in claim 10 in which the light reflected by the multilayer films at certain angles of view is in the infrared region of the electromagnetic spectrum.
12. A security device as claimed in any one of the preceding claims further comprising a machine readable element.
13. A security device as claimed in claim 12 in which the machine readable element is in the partial light absorbing layer or dye in the second film.
14. A security device as claimed in claim 12 in which the machine readable element is in an independent layer.

-17 -
15. A security device as claimed in any one of claims 12 to 14 in which the machine readable element comprises a fluorescent, phosphorescent, infrared absorbing, thermochromic, photochromic, magnetic, electrochromic, conductive or piezochrornic material.
16. A security device as claimed in any one of the preceding claims further comprising a layer of adhesive on one or both sides of the device.
17. A security document comprising a substrate and security device as claimed in any one of the preceding claims.
18. A security document as claimed in claim 17 in which the security device is applied to a surface of the substrate.
19. A security document as claimed in claim 17 in which the security device is at least partially embedded in the substrate and visible in windows in at least one surface of the substrate.
20. A security document as claimed in any one of claims 17 to 19 comprising a voucher, passport, banknote, cheque, certificate or other document of value.
21. A security document as claimed in any one of claims 17 to 20 in which the document is printed with identifying information and designs formed by the reflection of light from the much larger films of. the security device are linked to the identifying information.

-18 -
22. A security document as claimed in claim 21 in which the link is by content and/or registration of the designs.
23. A security device substantially as hereinbefore described with reference to and as shown in the accompanying drawings.